Carbon nanotubes as linewidth standards for SEM & AFM

Details Carbon nanotubes as linewidth standards for SEM & AFM Carbon nanotubes as linewidth standards for SEM & AFM

2000-12-04

2003-07-15

Noland, Thomas P. (Department: 2856)

Measuring and testing

Instrument proving or calibrating

Roughness or hardness

C029S701000, C438S048000

Reexamination Certificate

active

06591658

ABSTRACT:

TECHNICAL FIELD
The present invention relates to nanometerology and in particular to calibration methods, calibration standards, and calibration systems for nano-scale measuring devices.
BACKGROUND OF THE INVENTION
Nano-scale measuring devices, which include electron microscopes and scanning probe microscopes (SPMs), are devices capable of making measurements with a spatial resolution of 10 nm or less. The ability to resolve small differences means that these devices are very precise. A nano-scale measuring device may be able to measure, when comparing objects, differences in feature size on the order of nanometers, or even Angstroms. Yet a very precise measurement is not necessarily an accurate one. Accuracy refers to the correctness of a measurement in absolute terms.
To obtain measurements that are accurate as well as precise, calibration of the measuring device is generally required. Calibration usually involves measurements made on samples having known dimensions or otherwise well characterized. Calibration is intended to ascertains systematic errors so that they may be taken into account in interpreting device measurements Calibration measurements are subsequently used to correct measurement data to remove or mitigate systematic errors.
The sources of systematic error and the number of measurements required to calibrate a device depend on the type of device. Consider an atomic force microscope (AFM)
200
(a type of SPM), illustrated in
FIG. 1
a.
The atomic force microscope
200
measures the topography of a sample
210
. A controller
220
directs a sample stage
230
to move the sample
210
past a detection tip
240
, which is mounted on a cantilever
250
. As the sample
210
moves past the detection tip
240
, the sample stage
230
provides the controller
220
with feedback regarding the position of the sample stage
230
and an interferometer
260
provides the controller
220
with data regarding the height of the cantilever
250
. The controller
220
combines the sample position data, the tip position data, and data regarding the tip shape to reconstruct the sample topography.
Each source of data to the controller
220
may contain systematic error. With respect to the sample position data, for example, the feedback provided by the sample stage
230
may be the number or turns in a screw that controls the position of the sample stage
230
. Relating the number of turns in the screw to the actual position of the sample may introduce systematic error. With respect to tip movement data, the readings from the interferometer
260
may not have a one-to-one correspondence with the actual movement of the detection tip
240
. For example, the point
252
on the cantilever
250
on which the interferometer
260
is focused moves at a different rate than the detection tip
240
if the point
252
is a different distance from the cantilever pivot point
254
. Finally, because the movement of the detection tip
240
depends on the shape of the detection tip
240
as well as the topography of the sample
230
, errors in data regarding the shape of the detection tip
240
lead to errors in the topography data produced by the AFM
200
.
An ideal calibration of a nano-scale measuring device, one that would take into account all sources of systematic error regardless of their source, would involve making measurements on a series of calibration standards with precisely known shapes and sizes spanning the range of shapes and sizes of objects to which the instrument is to be applied. The accuracy with which the dimensions of the standards are known is ideally greater than the precision of the device being calibrated, an order of magnitude greater if possible. For nano-scale measuring devices, obtaining standards having the ideal precision and range of shapes and sizes has proven difficult, if not impossible. Available standards are not characterized with the desired degree of accuracy and are not suitable for addressing all sources of systematic error.
For example, Bartha et al., U.S. Pat. No. 5,665,905 (the “Bartha patent”) provides a calibration standard for calibrating the shape of a SPM tip. The standard has a trench and a raised line, the trench having a width intended to be equal to the thickness of the line. The widths of the trench and raised line are about 500 nm and the uncertainty of these widths is at least about +/−1 nm due to the roughness of the surfaces. The SPM is calibrated by moving the calibration standard past the SPM detection tip. The measured width is based on the SPM measurements and the movements of the sample stage.
First the SPM scans a trench. Prior to entering the trench, the SPM measures a first height value, the height outside the trench. As the scanning tip enters the trench, the SPM measurement undergoes a first transition after which the SPM measures a second height, the height at the base of the trench. As the scanning tip leaves the trench, the SPM measurement undergoes a second transition as the SPM returns to measuring the first height. The measured width of the trench is considered to be the length over which the sample stage, or SPM tip, moves from the beginning of the first transition to the end of the second transition. It is expected that this measured width is less than the actual width of the trench due to the finite tip width. The first transition does not begin until a trailing portion of the tip enters the trench. The second transition begins as soon as the leading portion of the tip leaves the trench. Therefore, the measured width of the trench is approximately the actual width of the trench minus the tip width.
To complete the calibration, the raised line is scanned. As the scanning tip reaches the line, the SPM measurement undergoes a third transition as the SPM goes from measuring a third height to a fourth height. As the scanning tip passes the line the SPM measurement undergoes a fourth transitions in which the SPM returns to measuring the third height. The measured width of the line is considered to be the length over which the sample stage, or the SPM tip, moves from the beginning of the third transition to the end of the fourth transition. It is expected that this measured width is greater than the actual width of the line due to the finite tip width. The third transition begin when the leading portion of the tip reached the line. The fourth transition ends when the trailing portion of the tip passes the line. Therefore, the measured width is approximately the actual line width plus the tip width.
The two measurements may be combined to calibrate the tip width. Assuming that the actual line and trench widths are equal, the measure width of the line is approximately two tip widths greater than the measured width of the trench. The tip width is calculated by taking the difference between these two measurements and dividing by two. Applying the calibration consists of adding the calculated tip width to measured trench widths and subtracting the calculated tip width from measured line widths.
A number of factors limit the accuracy of this calibration. For example, there is the uncertainty in the line and trench widths. The calibration provides only one number, the tip width, but the tip shape may be relatively complex. Other potential sources of systematic error not taken into account include: inaccuracy in the stage or SPM tip movement measurements, differences between the line and trench widths, and difficulty in identifying the exact starts and ends of the height measurement transitions.
Another SPM calibration standard and calibration method is provided by Bayer et al., U.S. Pat. No. 5,578,745 (the “Bayer patent”). The Bayer patent calibration standard is formed by making several sharp groves in a single crystal material. The groves have well defined slopes that meet to form relatively sharp peaks. The peak radii are measured with a scanning electron microscope (SEM) or transmission electron microscopy (TEM) and are said to be known within +/−0.5 nm. SPM tip widths are calibrated by scanning across

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